How updates happen for struts, workareas, and screen/xinerama regions/edges:
  One of three things causes meta_window_update_struts to be called
    (a) initial window map (window.c:meta_window_new_with_attrs())
    (b) update of _net_wm_strut* properties (window.c:process_property_notify())
    (c) screen resizes (e.g. via xrandr; from screen.c:meta_screen_resize_func())
  meta_window_update_struts (MetaWindow *window)
    - Gets new list of struts from window properties
    - Makes sure window doesn't single-handedly fill the screen
    - records new struts if different and calls invalidate_work_areas()
  invalidate_work_areas ()
    - Calls meta_workspace_invalidate_work_area() for each workspace it's on
  meta_workspace_invalidate_work_area()
    - Cleans out all strut lists
    - queues all windows for resizing
    - Calls meta_screen_queue_workarea_recalc (workspace->screen);
  meta_screen_queue_workarea_recalc()
    - Adds set_work_area_idle_func() as an idle handler

  set_work_area_idle_func()
    - Calls set_work_area_hint()
  set_work_area_hint()
    - Calls meta_workspace_get_work_area_all_xineramas()
    - Sets _NET_WORKAREA property
  meta_workspace_get_work_area_all_xineramas()
    - Calls ensure_work_areas_validated()
  ensure_work_areas_validated()
    - Loops over xineramas
      - Loops over windows, then struts:
        - Adds struts to list first time through xinerama loop
        - Find the amount of the strut on the given xinerama for <dir>_strut
        - Just max the amount of the strut with the all_<dir>_strut
      - Makes sure there's a non-empty xinerama workarea
      - Record the xinerama workarea
    - Make sure there's a non-empty screen workarea
    - Record the screen workarea
    - Cache the spanning rects for the screen and xinerama regions
    - Cache the screen and xinerama edges

  Alternatively to all the above, if the idle function for the screen
  has not yet fired, constraints.c:setup_constraint_info() can call
  either workspace.c:meta_workspace_get_onscreen_region() or
  workspace.c:meta_workspace_get_onxinerama_region() which in turn
  call workspace.c:ensure_work_areas_validated().

Meaning of related functions that might be difficult to tell apart:
  screen.c:meta_screen_get_current_xinerama ()
    - Finds out which xinerama the mouse is on with an XQueryPointer
  window.c:meta_window_get_work_area_current_xinerama()
  window.c:meta_window_get_work_area_for_xinerama()
  window.c:meta_window_get_work_area_all_xineramas ()
    - All three are for finding the intersection of workareas across
      multiple workspaces so that placement of windows can be
      determined in such a way that they remain in the workarea for
      all workspaces that they are on.